Accidentally set shortcut for vertex select to left mouse. How do I undo this?
If you didn't after saved user preferences, it's not permanent, just close and reopen blender.

If you did it, open user preferences, then switch to "input" panel.

Then open "3d view" > "mesh" and at the bottom of the expanded list find "select mode" with "mouse" listbox value and "left mouse" setting shown.

Click on the "x" button. Done (save now the user preferences).